Micro-electro-mechanical system (MEMS) sensor and method for making same

ABSTRACT

The present invention discloses an MEMS sensor and a method for making the MEMS sensor. The MEMS sensor according to the present invention includes: a substrate including an opening; a suspended structure located above the opening; and an upper structure, a portion of which is at least partially separated from a portion of the suspended structure; wherein the suspended structure and the upper structure are separated from each other by a step including metal etch.

This is a Divisional of a co-pending application Ser. No. 12/391,551,filed on Feb. 24, 2009. BACKGROUND

1. Field of Invention

The present invention relates to a micro-electro-mechanical system(MEMS) sensor, in particular to a MEMS pressure sensor; the presentinvention also relates to a method for making such MEMS sensor, which iscompatible to standard CMOS process.

2. Description of Related Art

MEMS devices are used in a wide variety of products; examples of suchapplications are absolute and relative sensors such as blood pressuresensor and micro-acoustical microphone. U.S. Pat. Nos. 6,012,336;6,536,281; 6,928,879; 7,121,146; 6,743,654; and 7,135,149 are relevantart, but the processes employed in these prior art either are toocomplicated or require equipment or materials which are not compatibleto or not standard in CMOS process. A MEMS sensor having a structurewhich can be manufactured by a standard CMOS process is desired in thisindustry.

SUMMARY OF THE INVENTION

It is an objective of the present invention to provide a MEMS sensorwhich can be manufactured by a process fully compatible to the presentstandard CMOS process.

It is another objective of the present invention to provide a method formaking such MEMS sensor.

In accordance with the foregoing and other objectives of the presentinvention, from one aspect, the present invention discloses a method formaking a MEMS sensor, comprising: providing a substrate; forminginterconnection on the substrate, part of the interconnection forming anetchable structure separating at least a portion of a suspendedstructure of the MEMS sensor from the rest of the MEMS sensor; etchingthe back side of the substrate to expose the etchable structure; andetching the etchable structure.

From another aspect, the present invention discloses a MEMS sensorcomprising: a substrate including an opening; a suspended structurelocated above the opening; and an upper structure, a portion of which isat least partially separated from a portion of the suspended structure;wherein the suspended structure and the upper structure are separatedfrom each other by a step including metal etch.

In the above MEMS sensor and the method, the back side of the substratecan be sealed by hermetical package so that the MEMS sensor becomes anabsolute sensor. The material for hermetical package for example can besilicon or glass.

The sensor can further comprise a guard ring encompassing the suspendedstructure.

It is to be understood that both the foregoing general description andthe following detailed description are provided as examples, forillustration and not for limiting the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood with regard to the followingdescription, appended claims, and accompanying drawings.

FIGS. 1A-1E show an embodiment according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The drawings as referred to throughout the description of the presentinvention are for illustration only, but not drawn according to actualscale.

Referring to FIG. 1A for the first embodiment of the present invention,a zero-layer wafer substrate 11 is provided, which for example can be asilicon wafer so that the process is compatible with a standard CMOSprocess. Next, transistor devices can be formed by standard CMOS processsteps as required (not shown), followed by deposition, lithography andetch steps to form interconnection including a contact layer 12 a, metallayers 13, and via layers 12 b. A dielectric layer 14 is provided toisolate the metal patterns of the layers where there should not beconnection. Furthermore, a bond pad pattern 15 is formed on the topmostmetal layer, and a passivation layer 16 is formed on top of the overallstructure. In one embodiment, the contact layer 12 a and the via layers12 b can be made of tungsten; the metal layers 13 can be made ofaluminum; and the dielectric layer 14 can be made of oxides such assilicon dioxide. Other conductive or dielectric materials can be used toreplace what are suggested above, and the structure can include more orless number of metal layers.

In the shown structure, between the innermost first metal layer 13 andthe outermost metal structure (item 30 of FIG. 1C, referred to also asthe “upper structure” hereinafter), an etchable structure 20 is formedwhich includes a portion of the contact layer 12 a, metal layers 13, anda via layer 12 b. The purpose of the etchable structure 20 is to definea suspended structure of the MEMS device, which will become clearer asreferring to the following process steps. The outermost metal structurecan either provide a guard ring function to protect the circuitry (notshown) from etch damage or moisture, or act as an upper electrode (to beexplained later).

Referring to FIG. 1B, the back side of the substrate 11 is subject to anetch, such as an ICP (Inductively Coupled Plasma) DRIE (Deep ReactiveIon Etch), to forming an opening exposing the etchable structure 20.

Referring to FIG. 1C, the etchable structure 20 is etched away to leavea space 20 which separates the suspended structure 40 from the rest ofthe MEMS device. The etch can for example can be wet etch by sulfideacid and hydrogen peroxide solution. Note that the term “separate” inthe context of this specification means “at least partiallydisconnected” but does not have to be “totally disconnected”. In aposition not shown in this figure, the suspended structure 40 is stillconnected with the zero-layer wafer substrate 11. In one application,the suspended structure 40 functions as a lower electrode and fixed tothe substrate; it does not deform during operation of the sensor. On theother hand, the upper structure 30 functions as an upper electrode; itsuppermost metal layer 13, or a composite layer of the uppermost metallayer 13, part of the dielectric layer 14 and part of the passivationlayer 16, form a thin film structure which elastically deforms duringoperation of the sensor according to acoustical or air pressurevariation. The deformation causes a change of the capacitance.

After the etchable structure 20 is etched, as shown in FIG. 1D, part orall of the passivation layer 16 can be removed by lithography and etch,to open the bond pads 15 at the topmost layer of the interconnection. Ifthe upper structure 30 is used as an upper electrode and it is desiredto reduce the thickness of the upper electrode thin film, thepassivation layer 16 above the upper electrode, or together with thedielectric layer 14 above the upper electrode, can removed in this step.

If what is intended to make is a relative sensor such as acousticalsensor or accelerometer, the process can stop here. If what is intendedto make is an absolute sensor, referring to FIG. 1E, preferably, ahermetical package is formed on the back side of the zero-layer wafersubstrate 11 by a sealing material 17, which for example can be siliconor glass.

The foregoing process is superior to the prior art because it is fullycompatible with standard CMOS process. By such process, a MEMS deviceand CMOS devices can be made on the same chip in a CMOS fabconveniently.

Although the present invention has been described in detail withreference to certain preferred embodiments thereof, the description isfor illustrative purpose and not for limiting the scope of theinvention. One skilled in this art can readily think of othermodifications and variations in light of the teaching by the presentinvention. For example, the materials, number of metal layers, etch inthe shown embodiments are provided as examples; they can be modified inmany ways. As another example, the outermost metal structure does nothave to form a guard ring. Therefore, all such modifications andvariations should be interpreted to fall within the scope of thefollowing claims and their equivalents.

1. A method for making a MEMS sensor, the method comprising: providing a substrate; forming interconnection on the substrate, the interconnection including a metal structure, a suspended structure, and an etchable structure, the etchable structure at least separating a portion of the metal structure from a portion of the suspended structure; wherein the etchable structure including metal layers; etching the back side of the substrate to expose the etchable structure; and etching the etchable structure to form a space between the suspended structure and the metal structure.
 2. The method of claim 1, wherein in the step of etching the back side of the substrate, the metal structure and the suspended structure are not exposed.
 3. The method of claim 1, further comprising: sealing the back side of the substrate by a hermetical package.
 4. The method of claim 3, wherein the hermetical package includes a material of silicon or glass.
 5. The method of claim 1, further comprising: forming a passivation layer on the interconnection, and etching the passivation layer to expose a bond pad which is electrically connected with the interconnection.
 6. The method of claim 1, wherein the interconnection forms a guard ring encompassing the suspended structure.
 7. The method of claim 1, wherein the interconnection includes metal layers made of a material including aluminum, and contact and via layers made of a material including tungsten, and wherein the step of etching the etchable structure includes using sulfide acid and hydrogen peroxide solution.
 8. The method of claim 1, wherein the substrate is a silicon substrate, and the step of etching the back side of the substrate includes ICP etch (Inductively Coupled Plasma etch).
 9. The method of claim 1, wherein the metal structure and the suspended structure form upper and lower electrodes, respectively.
 10. The method of claim 5, further comprising: etching to remove the passivation layer above the metal structure.
 11. The method of claim 1, further comprising: forming a dielectric layer on the substrate, and etching to remove the dielectric layer above the metal structure. 